This invention relates to x-ray beam therapy. In particular, it relates to computer tomographic scanner and their use in delivering a therapy dose of x-rays.
A computer tomography ("CT") scanner can localize precisely many tumors and other lesions in the brain. After the CT scan is obtained, a patient is taken to another location for treatment with high energy photons from a gamma knife or a linear accelerator.
The proper alignment of the patient in the radiation field requires that the lesion in the brain must be located with respect to external markers provided by a stereotactic frame. This is attached to the skull by surgical screws. Mounting the frame, locating the lesion with respect to the coordinates and then directing the therapy beam is a difficult and time consuming task.
Current stereostatic therapy procedures are time consuming an unpleasant for the patient. The stereostatic locating operation alone is relatively traumatic since holes are drilled into the skull while the patient receives only a local anesthetic.
Once the device is absolutely secure on the patient's head, the patient is transported to a diagnostic x-ray room in order to localize the brain tumor with respect to setting the coordinates on the stereostatic device. Finally the patient is transported to a third room for the actual radiation treatments by the gamma knife (an array of cobalt-60 sources focused at a point) or other high energy photon source such as the linear accelerator.
The knife is stable but expensive and inflexible in the volume irradiated. Linear accelerators tend to wobble during rotation, limiting precision, and are limited to a single arc of rotation. Moreover, the large exit doses of high energy photon beams place at risk relatively large volumes of normal tissue.
CT scanners are well known for use as diagnostic tools. For this purpose, the CT scanner uses an x-ray source mounted on a gantry which can rotate in an arc about a subject located on a couch at about the axis of rotation. A gantry can be tilted about the axis of rotation such that different slices of image can be obtained of the subject. CT scanners have developed for different diagnostic purposes, but in the Applicant's knowledge, have not been used for therapy purposes.
Different scanning mechanisms have been employed in CT scanners. Commonly, the first generation scanner used a pencil beam which moved linearly across the entire head of the subject in a parallel fashion to generate a first series of data at a scan 45.degree. position. Thereafter, the x-ray tube was moved to the scan 90.degree. position, 45.degree. away and the same pencil beam was then directed linearly across the entire head. This was repeated at the scan 135.degree..
In a second generation CT scanner, the beam was replaced with a narrow fan beam but essentially the linear direction of the beam across the entire patient's head was effected in each of three scan positions.
In the third generation CT scanner, a wide fan beam was used sufficient to encompass the entire width of the patient. The gantry moved in a rotating action continuously. The fourth generation scanner permits the tube to rotate through an arc of 360.degree. with a wide fan beam encompassing the entire width of the patient's head.
Although a fan beam is useful for obtaining a high volume of data for multiple detectors quickly, it is not appropriate for use in therapy applications. Additionally, the x-ray tube is in the orthovoltage range at about 120 KVP to about 300 KVP. Such a voltage range is conventionally suitable only for diagnostic purposes.
When x-ray therapy is effected, it is currently regarded as desirable to use a voltage range in the order of 2 to 6 MEV since this can deliver a high voltage dose to a targeted tumor. In addition, the skin dosage from such a beam is relatively lower than that from a orthovoltage beam. High KVP x-ray systems are poor for visualization since there is too little contrast with the different tissue. The low KVP beam, on the contrary, is effective for diagnostic purposes since there is good contrast. The voltage, however, is unsuitable for effective treatment of deep tumors with the conventional 2 or 3 fixed fields.
There is accordingly a need to provide an improved system for therapy which avoids the complex mechanical systems currently in use and which can use the benefits of diagnostic CT scanners.